One terminal capacitor interface circuit

Abstract

A differential capacitor one terminal capacitor interface circuit for sensing the capacitance of first and second capacitors includes a differential integrating amplifier having first and second summing nodes and an input common mode voltage; and a switching circuit for charging a first capacitor of said differential one terminal capacitor to a first voltage level and a second capacitor of said differential one terminal capacitor to a second voltage level in a first phase, in a second phase connecting said first capacitor to said first summing node and said second capacitor to said second summing node of said amplifier to provide first and second output changes substantially representative of the difference between said first and second voltage levels and said input common mode voltage, in a third phase charging said first capacitor to said second voltage level and said second capacitor to said first voltage level, and in a fourth phase connecting said first capacitor to said second summing node and said second capacitor to said first summing node of said amplifier to provide third and fourth output changes substantially representative of the difference between said first and second voltage levels and said input common mode voltage, the combined first, second, third and fourth changes representing the capacitance of said first and second capacitors substantially independent of said input common mode voltage.

Description

RELATED APPLICATIONS [0001] This application is a divisional patent application of, and claims the benefit of and priority to U.S. patent application Ser. No. 11 / 370,764 filed Mar. 8, 2006, which claims the benefit of and priority to U.S. Provisional Application No. 60 / 660,415 filed Mar. 9, 2005, all of which are incorporated by reference herein.FIELD OF THE INVENTION [0002] This invention relates to an interface circuit for sensing capacitance of a one terminal capacitance, and more particularly to such an interface circuit useful in sigma delta modulators and converters and charge amplifiers or capacitance to voltage converters. BACKGROUND OF THE INVENTION [0003] A large number of sensors are capacitive—the physical effect being measured is translated into a change in capacitance which can then be measured electrically. In many cases the two terminals of the sensor capacitor are floating, that is, neither are at a fixed potential. This is very convenient when interfacing the senso...

AI Technical Summary

Benefits of technology

[0009] It is a further object of this invention to provide such an improved one terminal capacitor interface circuit which can cancel amplifier offset error and 1 / f low frequency noise.

[0020] In a preferred embodiment the differential integrating amplifier may include a control circuit for controlling the input common mode voltage at the summing nodes to be substantially equal to an applied reference voltage. The differential integrating amplifier may include a control circuit for controlling the output common mode voltage of the amplifier to be substantially equal to an applied reference voltage. The differential integrating amplifier may include a chopper circuit for selectively inverting the inputs and the outputs of the differential integrating amplifier for canceling amplifier offset error and low frequency noise.

Problems solved by technology

This then limits the circuit topologies that can be used.

In all cases the variation in Vy will corrupt the charge being transferred from the sensor to the integrator, and will cause an error in the measurement of the sensor output.

With these single input integration circuits the integration amplifier offset error and 1 / f low frequency noise cannot be easily corrected.

Images